The present disclosure relates to medical imaging. More particularly, the disclosure relates to Z-shim compensation for magnetic resonance imaging (MRI).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0) applied along, for example, a Z axis of a Cartesian coordinate system, the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped,” into the x-y plane to produce a net transverse magnetic moment Mt. A NMR signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image or produce a spectrum.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space.” Typically, a region to be imaged is scanned by a sequence of measurement cycles in which gradients vary according to the particular localization method being used. Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) that have the same direction as the polarizing field B0, but which have a gradient along the respective x, y, and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified. The acquisition of the NMR signals samples is referred to as sampling k-space, and a scan is completed when enough NMR cycles are performed to adequately sample k-space. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Functional magnetic resonance imaging (fMRI) provides an approach to study neuronal activity. Conventional fMRI detects changes in cerebral blood volume, flow, and oxygenation that locally occur in association with increased neuronal activity that is induced by functional paradigms. This physiological response is often referred to as the “hemodynamic response.” The hemodynamic response to neuronal activity provides a mechanism for image contrast commonly referred to as the blood-oxygen level dependent (BOLD) signal contrast. An MRI system can be used to acquire signals from the brain over a period of time. As the brain performs a task, these signals are modulated synchronously with task performance to reveal which regions of the brain are involved in performing the task. The series of fMRI time course images must be acquired at a rate that is high enough to see the changes in brain activity induced by the functional paradigm. In addition, because neuronal activity may occur at widely dispersed locations in the brain, a relatively large 3D volume or multi-slice volume must be acquired in each time frame.
Unfortunately, fMRI imaging of the brain acquired using gradient-echo echo-planar imaging (GE-EPI) consistently suffers adverse effects from magnetic field inhomogeneity in regions close to nasal sinuses and ear canals. The presence of both air and tissue causes magnetic susceptibility variation at the boundary of the two regions, resulting in both geometric distortion and signal loss. Signal loss occurs due to phase accumulation, which pushes the signal outside the slice encoding region. This signal can be recovered back into the imaging plane via Z-shim methods, where a Z gradient is applied across the slice prior to data acquisition. This is commonly implemented as a two-shot protocol, where each final image is formed through a combination of two Z-shimmed images, each emphasizing signal regions that are “dark” in the complementary image. However, doing so necessarily reduces the temporal resolution by half.
Single-shot Z-shim methods are attractive because data can be acquired for both components of the final image after a single RF excitation pulse. However, these methods commonly require longer EPI echo-trains, which can increase susceptibility effects resulting in increased geometric distortion.
It would be desirable to have a system and method for controlling or compensating for magnetic susceptibility variations within a subject to be imaged using an MRI system, such as during an fMRI study.